In Children of Men, For some reason which we're not really told, the world is in total chaos.
Is there any background info anywhere about why it is so, and how does it correlate to the infertility problem?
If the world was suddenly told, "Humanity will die out in approximately 50 years," with irrefutable evidence, societal structure and the world economy would pretty quickly crash from panic.
I don't remember if this is ever explicitly said, but there is a tone of global panic throughout the movie. The exact details of how aren't important, but the why is implied throughout the movie: people taking advantage of each other. That is a theme echoed throughout the movie, and is meant to contrast Theo's ultimate selflessness.